1. Field of Invention
The present invention relates to a method of detecting icing and icing growth rates on an aircraft through measurements of the aircraft's flight performance.
2. Description of Related Art
A major difficulty with multi-mode aircraft is that icing sensors are difficult to effectively and safely position. For example, the Bell/Agusta 609 and the Osprey V-22 aircraft will operate in both a hover mode, like a conventional helicopter, and in a fixed wing mode, like most normal aircraft. Unfortunately, this characteristic can make it difficult to know where to position icing sensors since the instruments will be subjected to totally different dynamic conditions depending upon whether the aircraft is hovering, flying fixed wing, or in some mode in between, as might occur during take off. In order to cope with this dilemma an effort was made to determine if useful information could be obtained from the general performance of the aircraft under icing conditions to determine, or at least supplement, traditional icing sensor output to anticipate icing conditions and take appropriate corrective action before serious control and lift problems occur.
Some aspects of the present invention are known in the prior art. Previous investigations by Leigh Instruments for the U.S. Army were directed towards a system for inferring the rate of ice accretion on a UH-1 rotor, through measurements of torque increases during periods of flight in known icing conditions. (See Macmillan, R., "Advanced Icing Severity Level Indicating System (AISLIS)," USAAVSCOM TR-86-D-7, December 1986.) This system was only partially successful, due to its reliance on significant pilot input for aiding the calculations performed by the device's internal UH-1 performance model (such as providing cargo weight and aircraft drag data), and for its use of relatively simplistic logic in gauging the type of icing present when torque increases were present in flight.
Related system monitoring devices are already part of, for example, the V-22 military aircraft. A similar combination of direct and indirect measurements are currently used in the V-22 Osprey's Central Integrated Checkout (CIC) and Vibration, Structural Life and Engine Diagnostic (VSLED) systems for maintenance and usage monitoring. (See Augustin, M. and Middleton, G., "A Review of the V-22 Health Monitoring System," Proc. 45th AHS Annual Forum, Boston, Mass., May 1989). This latter system acts as a terminal on the V-22's 1553 databus and uses aircraft parameters in conjunction with dedicated accelerometers to determine both aircraft flight load spectra and provide data for track and balance maintenance. A key advantage of the system is the elimination of the requirement for a large collection of dedicated and specialized instrumentation to perform this function. Monitoring logic for these systems primarily uses Boolean comparisons between measurements and reference points to determine system faults, plus capability for storage of time histories of out-of-condition data for subsequent post-flight analysis and maintenance activities.
Current state-of-the-art in aircraft icing detection typically incorporates discrete sensors located in strategic positions about the aircraft fuselage or wing structure in order to directly sense the amount of icing growth present at the sensor location. These sensor data may then be used to infer the ice growth at other locations on the aircraft if sufficient test data are available for extrapolation purposes. Such approaches are limited by the availability of aircraft structure to accommodate the given sensor, requirements to transfer power and data signals to each of these possibly remote sensors, and the need to have suitable data for proper interpretation of their associated output measurements.